Two ply printing paper and method of producing the same



Nov. 2, 1965 3,215,589

TWO FLY PRINTING PAPER AND METHOD OF PRODUCING THE SAME Filed Dec. 20,1962 H. H. HELLER ETAL 2 Sheets-Sheet 1 DRY W55 1965 H. H. HELLER ETAL3, 15,589

TWO FLY PRINTING PAPER AND METHOD OF PRODUCING THE SAME Filed Dec. 20,1962 2 Sheets-Sheet 2 7 To M/VD-UP United States Patent 3,215,589. TWOPLY PRINTING PAPER AND METHOD OF PRODUCING THE SAME Harold H. Heller andVictor D. Werner, Appleton, Wis.,

assignors to Kimberly-Clark Corporation, N eenah, Wis.,

a corporation of Delaware Filed Dec. 20, 1962, Ser. No. 246,134 3Claims. (Cl. 162124) This invention relates to printing papermanufacture. More particularly, the invention is primarily concernedwith printing paper for use as bookpaper, catalog paper, publicationpaper, newsprint, and the like, although it may be used advantageouslyin other paper products.

Paper adapted for high quality printing has customarily been heavilycalendered or has been coated followed by supercalendering. Suchtreatments have been necessary to achieve the level surface required forprinting and for adequate ink holdout. Coating, however, materiallyincreases the basis weight of a paper sheet; calendering andsupercalendering, in addition to reducing sheet thickness and opacity,render the sheet more brittle and harden the paper to such an extentthat printing quality is affected adversely.

In the absence of a coating, and frequently with coatings, wire formedwebs, as is well known in the art, tend to exhibit a definite two-sidedcharacteristic, that is, the wire side is much rougher than the feltside. In the art the minute variations of a paper surface are commonlytermed ridges and valleys and it is the extent of these variations in adirection perpendicular to the general plane of the sheet which is anindication of the roughness of the paper surface.

Mineral coated paper, while apparently level to the unaided eye,exhibits under the microcsope zones of differing characteristics both asto levelness and gloss; the surface includes high and low spots whichare rendered less prominent by supercalendering but at the expense ofdensity and of varying density since the high spots are compressed to agreater degree than the low spots. Gloss is also affected by thedistribution of pigment particles which likewise are compacted in thecoating surface by supercalendering operations. While such compactiondecreases the distance between individual particles of the pigment, thespread of particles still exists and contributes to nonuniform glosscharacteristics.

The production of light weight webs having a level and light reflectivesurface on one side, commonly termed machine glazed paper, is well knownin the paper manufacturing art. Such paper is produced commercially bypressing a wet cellulosic web against a polished drum surface and dryingthe web in contact with the surface; the smooth polished drier surfaceis reproduced on the side of the web adhering to the drier. Thisprovides the one highly glazed surface. The side of the web opposite thedrier surface is rough initially. Consequently, the dried sheet isdistinctly two-sided. Accordingly, such sheets have found no utility inthose applications requiring high quality printing on opposed sides ofthe sheet.

This present invention involves a new approach to the overcoming ofprior art limitations in connection with printing paper manufacturewhile utilizing some of the above-noted principles of the paper art.Essentially, the invention contemplates the uniting of machine glazedwebs, without loss of web glaze, into a multi-ply sheet of lighterweight than an integral sheet having corresponding printing properties.While duplex and multi-ply sheets have been proposed for newspaper andcatalog printing and the like, such efforts have commonly involvedlamination in the wet state and/or calendering and supercalenderingoperations which prevent the at- I 3,215,589 Patented Nov. 2, 1965 icetainment of bulk and other desirable properties in the finished product.

In papermaking terminology bulk or apparent density is the term appliedto the paper basis weight in pounds per ream divided by the sheetcaliper or thickness. In effect, this apparent density is a measure ofthe air content of a sheet, and it is this density or bulk which is amaterial factor in printing quality. The basis weight as usedhereinafter is the weight of 500 sheets of 25"x38" and the caliper isthat of a single sheet.

It is a primary object of the present invention to provide improvedprinting paper which does not require, but in some respects mayadvantageously employ, calendering, coating, or light supercalendering,and which paper is of low apparent density (high bulk), gloss, anddelamination resistance.

It is a further object of the invention to provide an improved processfor the manufacture of printing paper, in particular an improved processwhich may utilize and upgrade relatively inexpensive cellulose fiberfurnishes.

We have found that such objects may be attained, and disadvantages ofprior art methods in the production of high quality printing paper maybe overcome by carrying out the following generally stated method.Initially there is applied to a machine glazed, dried, sized web on itsrough surface side a thin film consisting wholly or primarily of aqueousadhesive; there is then advanced substantially immediately after theapplication of the adhesive a second and similar machine glazed web intocontact with the first web. This second web is so presented to the firstweb that their rough sides are confronting. The webs are superposedwithin sufiicient time to inhibit penetration of moisture from theadhesive to the machine glazed surfaces. The superposition step isfollowed or accompanied by pressing the webs together under relativelylight pressure so that the webs interlock due to their roughenedsurfaces and contacting areas; ridge areas of the opposed rough surfacesare bonded together by the adhesive while valley areas of the surfacesare substantially unbonded. The duplex sheet is then dried as requiredto reduce the moisture content to that which it would normally haveunder the ambient conditions present. The composite sheet so formed isnot densified to any material degree and, in addition, exhibits airspaces which enhance the opacity of the sheet.

In accordance with a preferred embodiment of the process of ourinvention, two machine-glazed webs are first formed and dried on aYankee paper machine. Such machines are customarily employed in themanufacture of light weight paper, for example, tissues; acharacteristic of the Yankee is a large drier in the form of animperforate smooth-surfaced heated drum some 8 to 12 feet in diameter.Important considerations in connection with webs appropriately formed onsuch machines are that each web has one very level surface. The presenceof web gloss or light reflectance capacity, desired in some printingpapers, is of itself incidental. It is the levelness of the surfacecombined with resiliency of the web (bulk) which is of concern to theprinter. The characteristics of resilience and levelness are necessaryin order that skips in ink application may be avoided. While levelnessmay be achieved in large measure by calendering the paper or bysupercalendering a coated sheet, the gain is usually accompanied by amaterial decrease in paper thickness (directly related to bulk).Additionally, average density and nonuniformity of density increases dueto the differential compression of those incremental web surface areashaving high spots relative to web surface areas having low spots. Theglazed webs, in contrast, since they have a surface formed on the Yankeewhen the fibers of the web are wet and conformable to the smooth driersurface, are characterized by materially less densification, are nottransparentized as frequently occurs in dry calendering operations, andexhibit on the one side levelness of surface. The surface finish of websproduced in accordance with the invention is indicated by Ingersollglarimeter readings of 40-42 or more and not less than 38.

The stock or furnish for the webs may be any cellulos'ic furnishconventionally employed in producing machine glazed webs of high qualityand is suitably of a character termed a slow stock; that is, of aCanadian freeness of less than about 300'. Preferably, for economicalreasons the furnish for the webs in the present instance may contain alarge proportion of groundwood; such groundwood inclusion is alsoeffective in aiding attainment of a Well filled surface of the sheet andin achieving resultant sheet opacity. Ground'wood alone does not producea sufficiently strong sheet and the furnish therefore includes sulfiteor kraft pulp in significant quantity. Usually the groundwoodpercentage, to provide for opacity in the sheet, as well as economy, isbetween about 30 to 70 percent or more by weight of the furnish.

The machine glazed surfaces of the webs are sensitive to water andsuffer some loss of levelness and of gloss if wetted. -The loss oflevelness is occasioned by swelling of the fibers, particularly theswelling of surface fibers out of their original plane. These fibers maybe dried back under some pressure to provide substantially the originallevel surface; the gloss characteristic is usually impaired to someextent. To assist in avoiding web penetration and material disruption ofthe level surface during lamination of the webs to form the printingsheet, the furnish contains a size. Sizing also strengthens the web andis employed in furnishes which form webs which are to be machine glazedto aid passage of the light weight web through lamination machine draws.In the practice of this invention the usual size treatments areeffective but rnost suitably, the size, as noted hereinafter, isformulated to be receptive to the adhesive employed and to aid smoothspreading of the adhesive film.

I The formation of the 'web, that is, the lay of the cellulosic fibersis an important consideration both as to penetration of the web byadhesive during manufacture and as to opacity of the composite sheet. Ifthe web absorbency and porosity are not closely controlled andcorrelated with the fluidity of the adhesive and application nippressure, the adhesive material, as well as the aqueous content of theadhesive composition, may be forced through the rough web surfaces tothe glazed sides to impair the glaze and produce a nonuniform surfacelevel by swelling of surface fibers. We have found that pressing of eachof'the webs on the Yankee drier drum and drying on the drum not onlyproduces the desire-d substantially planar surface but the compaction offibers of the sheet on the side of the web adjacent the drier duringmanufacture is at a maximum and the fibers, through the thickness of theweb body to the opposite exposed and rough side, are less denselycompacted. This provides for our purpose desirable density conditions ineach web.

' The bonding aqueous adhesive coating compositon, however, should wetthe sized web surface sufiiciently to spread readily. Suitably, theadhesive, as noted, is desirably applied to the rough side of a firstone of the dried webs; it is then brought into contact with the roughside of the second dried web sufficiently quickly to cause the aqueouscontent of the adhesive to be distributed substantially uniformlybetween the two webs. Alternatively, a portion of the adhesive may beapplied to each Web. In either event the extent to which either web musttake up water from the adhesive is limited. Consequently, web wetting toa deleterious degree is avoided. If, before adhesive application, thewebs are thoroughly dried, the moisture content pickup by the webs maybe less than that commonly picked up by the webs under normal ambienttemperature and humidity conditions. However, some sheet drying isusually necessary.

The adhesive applied is also limited as to quantity to prevent completefilling of the spaces formed between the roughened opposed surfaces ofthe webs. The adhesive thus need be present only to the extent that agood bond is achieved between the Webs.

Spreading of the adhesive in a thin film aids in causing the adhesive tofollow primarily the ridges and valleys of the surface of the web towhich it is applied and complete filling of air spaces between the websis avoided. Suitably, the adhesive film is applied by reverse rollcoater or by intaglio printing.

Thus, in effect a balance exists between the absorbency of the webs andthe specific nature and quantity of coating applied. With the correctquantity of adhesive coating of a given nature and dry sized web isessentially only moistened and the web does not absorb sufiicient waterthrough the fibers to affect the glazed surfaces. Additionally, the twowebs do not form a plane interface and, due to the fiber masses of theridges and associated valleys, interlock to some degree. Nevertheless,the small size of the ridges and valleys and their close though openspacing permits a strong and satisfactory bonding action with a minimumof adhesive. The voids of spaces, being small in size and numerous,serve the dual purpose of increasing opacity and sheet bulk.

The solids content of the aqueous adhesives should be at least 30% andpreferably in excess of about 50%; such solids percentage is greaterthan would normally be required in an adhesive used simply to bond webstogether. Such solids content permits the adhesive material to enterinto the fiber structure of the webs to some extent but without causingmaterial penetration or hydration of the fibers. Such penetration isdesirably avoided beyond the extent needed for permanent adhesion, notonly to inhibit material fiber wetting and loss of surface smoothness,but also to prevent web brittleness, stiffness, and loss of opacity inthe finished product. The amount of adhesive coating commonly applied inthe practice of the invention to light weight webs should not exceedabout 4 pounds in the dry state and may be as low as 1 /2 pounds ofadhesive per ream of paper (25" x 38" X 500 sheets).

The adhesive of itself will not contribute to sheet opacity unless it ispigmented. In fact, the adhesive tends to lessen the opacity of thefinished sheet. When a high groundwood content, about 5070% or more ofthe furnish, is employed, there is no necessity for pigmentation.However, with webs made from conventional kraft and sulfite or even withlow groundwood, some pigmentation of the adhesive may be desirable forcertain printing papers. We have found that from about 510% by weight ofpigment (based on the dry weight of the adhesive), such as calciumcarbonate, TiO or the like may be employed without adversely affectingadhesion between the webs. However, since pigments tend to increase thebasis weight such are generally avoided where light weight is of primaryimportance; in such instances it is preferable to increase thegroundwood content of the webs.

The webs, to form a sheet useful in catalog, newsprint, and publicationgrade papers, each may have a basis weight in the range of to or poundsper 3300 square feet x 38 x 500 sheets), although for heavier printingpaper the webs weights may be greater. It is not essential that each oftwo webs forming a duplex sheet have the same basis weight, althoughthey should be in the same general range. Consequently, in the preferredembodment of the invention, the web basis weights will be between about10 and 15 pounds, and the dry adhesive weight will usually be betweenabout 1 /2 and 3 pounds per ream. In general, finished duplex sheetshaving an un-- coated basis weight in the 28 to pound range are most.particularly adapted for the practice of the invention since the websfor laminating may be formed readily at high speeds. Such sheets have athickness in the range of about 0.003" to about 0.005" at an apparentdensity of between 8,000 and 13,000.

The sheet formed by the composite of the webs and adhesive is withdrawnthrough a rolling nip and dried. Such drying, as already noted, evenwith aqueous adhesives, need only be very slight as the aqueous contentof the adhesive is not sufficient to materially raise the water contentof the sheet. Thus, if two 14-pound basis weight sheets are employedwith a 50% solids adhesive adapted to provide 4 pounds of adhesive dryweight on the finished sheet only about 4 pounds of Water need beevaporated or taken up by the web. This is about 14% on the dry fiberweight. The amount required to be evaporated is dependent upon theinitial moisture content of the web and am bient conditions. Thus, abone dry web would normally absorb about 6% of its own weight ofmoisture and, in the above example, only about 8% would be required tobe driven off.

While organic solvent systems rather than aqueous adhesives may beemployed, these are not particularly practical or economicallyadvantageous in the manufacture of paper on high speed machines eventhough wetting of the fibers is thereby inhibited. We have found that avariety of adhesives may be employed. Such include the animal glues,polyvinyl acetates, and particularly the dextrines.

The invention will be more fully understood by reference to thefollowing detailed description and accompanying drawings wherein:

FIG. 1 is a schematic illustration of apparatus useful in one embodimentof the invention;

FIG. 2 is a schematic illustration of a preferred arrangement ofapparatus in accordance with the invention;

FIG. '3 is a greatly enlarged representation of a paper web formed inaccordance with the practice of a method of the invention; and

FIG. 4 illustrates a modification of the apparatus arrangement for thepractice of the invention.

In the drawings corresponding numerals, where convenient, designatesimilar components. Also, since the web of FIG. 3 may be formed with anyof the apparatus arrangements of FIGS. 1, 2 and 4, such arrangements aredescribed first.

Referring initially to FIG. 1, the numeral 1 indicates a traveling paperweb of about 13 pound basis weight x 38 x 500 sheets, uncoated). ThisWeb, due to porosity, rough surface, and light weight, would not ofitself provide a suitable quality printing sheet. The web, as indicatedby the legend, is being passed from a wire and wet press section of aconventional Yankee drier machine on a felt F. As the web is passed overguide rolls 2 to the nip 3 it normally contains about solids and. 70%water.

An exemplary furnish from which the web 1 is formed contains about 60%by weight poplar groundwood refined to be free from fiber bundles; thebalance of the pulp is suitably bleached northern sulfite pulp refinedto a Canadian freeness of about 300. Accordingly, the freeness of thetotal pulp is considerably less than 300. Additionally, the furnishcontains about 6% by weight based on the fiber of fine White china clayas mineral filler; such serves to enhance sheet opacity. A sizingmaterial is also incorporated with the filler on the basis of about 1%on the fiber weight of the furnish; such sizing very suitably is a rosinsize containing about A of 1% of ammonia cut casein (basis on fiberweight) and is in the form of an emulsion; such sizing is eminentlysuitable to provide a machine glazed surface at production equipmentspeed. This furnish additionally contains sufficient alum to provide thepH of the furnish at about 5. In the preferred embodiment described, aretention aid may also be provided to assist retention of the clayduring the formation of the Web. Such retention aid is a mannogalactangum present to the extent of about /2% by weight based on the fiberweight.

The web 1 is directed to a nip 3 formed between a pressure roll 4 and adrier drum 5 having a highly polished surface. Roll 4 and drum 5 rotatein the direction indicated by the arrows. Pressure roll 4 exerts apressure of at least about 300 pounds per lineal inch on the web passingthrough the nip 3. Such pressure may very with the nature of the web butshould firmly urge the traveling web to the drier surface. The drieritself is steam heated internally and the temperature of operation ismaintained to provide a completed drying of the traveling web withinabout 250 of the drier circumference. In the present instance thetemperature of operation is about l200 F. drier surface temperature at aweb speed of about 2000 f.p.m. As the web is dried, steam is exhaustedthrough a hood 6 having an outlet 7.

The completely dried web designated generally at the zone 8 in FIG. 1 isdirected to a nip 9 between the drier drum and a second pressure roll10. Roll 10 is itself unheated.

Scraper blade 12, functioning in known manner, serves to maintain thesurface of drum 5 clean; the scraper does not contact the traveling webor formed sheet.

A second and similar previously formed web designated at 14 is directedalso at 2000 f.p.m. from a reel 15 by. guide roll 16 and backing roll 17to pressure roll 10 and to nip 9. Web 14 may be formed on the same orsimilar Yankee drier equipment, as described in connection with web 1.Pressure roll 10 may be driven independently of but in synchronism withdrum 5 to provide the same surface speed. In the course of its travel,the web 14- is lightly coated on its upper rough side with adhesive. Forthis purpose rolls 18 and 19 form an applicator nip of a reverse rollcoater having end dam 20. Adhesive is fed through inlet 21 and iscarried in the rotation of roll 19 in the form of a thin film 22 to theweb. Theadhesive film on the web is designated at 23, the representationof the film being somewhat exaggerated for purposes of illustration. Inessence, the coating adhesive is simply a thin sealer type of coat; toomuch adhesive is evidenced by the presence of a print roll type ofpattern. Higher laminating speeds are desirable, not only because ofeconomy but also because such tend to aid spreadingof the adhesive intothin film form.

It is to be noted that the web 1 in the nip 9 has a smooth glossy glazedsurface adjacent the drum 5 produced by drying of the web 1 on the drum.The opposite side of web 1, however, is in the present instance the feltside and rough. The web 14, having been similarly produced andappropriately reeled, has a lower glossy glazed Surface designated at G,and the upper surface which carries the adhesive film 23 is a roughsurface similar to the rough surface of the web 1. Thus, the webs 1 and14 meet in the pressure nip 9 with the adhesive film 23 between the nip0 is a relatively light pressure nip such that it does not materiallydensify the webs 1 and 14; about 50-100 pounds per lineal inch issatisfactory through, of course, the pressure at this zone is notparticularly critical since relatively dry webs are not readily subjectto fiber compression. The adhesive itself contains only a small quantityof water proportionally to web weight; accordingly, as the escapingmoisture is well distributed, the glazed sides of the webs 1 and 14 asthey pass through the nip formed by the smooth surfaced drum andpressure roll 10 are unaffected as to their levclness, gloss, anddimensional stability.

The adhesive is applied to the paper to the extent of 2 pounds per reamon a dry solids basis. The adhesive is a Borated Dextrin having a solidscontent of 50%; such adhesive is well known and commercially available.Accordingly, in the present instance only 2 pounds of Water per ream areeither absorbed by the moving webs or are evaporated off. If all of thewater content of the adhesive is dried off, this is only about 1poundper 1650 square feet of traveling web.

The viscosity of the 50% Borated Dextrin adhesive is about 3000 to 4000centipoises at a temperature of 70 7 F. as measured on the BrookfieldViscometer. It will be understood that it is desirable that the adhesivehave little penetration but that it wet the sized web in order to secureadequate adhesion and distribution.

As will be noted from FIG. 1, the combined adhered webs forming thesheet 24 pass for a short distance on the drier to permit adequatedistribution and escape of moisture. The sheet 24 is then guided fromthe drum 5 by roll 25 and is wound at 26.

It is to be particularly noted that the adhesive in the foregoingexample is applied to the preformed web; application of the adhesive tothe hot web intimately adhered to the Yankee drier surface would resultin immediate Web penetration and would inhibit removal of the web fromthe surface. In all instances the adhesive should be applied to a webwhich is in close contact with but not adhered to the lamination rollsurface.

While the apparatus of FIG. 1 may be operated in conventional manner,rolls 5, 18, 19 and 26 are usually driven and rolls 4, 10, and 17 aresimply rotatably mounted and driven by pressure contact. It will beunderstood that all rolls may be driven if appropriately synchronizedwith web speed.

In the preferred embodiment illustrated in FIG. 2, both glazed webs arepreformed and dried. As illustrated in FIG. 2, one dry web is fed fromreel 28 as web 30 under guide rolls 32 and around roll 34 to a pressurenip 36 formed between a heated drier roll 38 and a backing roll 40. Eachof these rolls 38, 40 are hard surfaced and preferably of steel,although backing roll 40 may suitably be of hard rubber. It is desirableunder all pressure conditions that the rolls be sufficiently hard toprevent high spots of the webs from being pushed through to the glazedsurfaces.

The second web is fed from reel 42 as web 43 over guide roll 44 andthrough an adhesive applicator nip 46. The adhesive applicator nip isformed by an applicator roll 47 and a backing roll 48. Roll 47 is anengraved roll, while roll 48 is hard surfaced and of rubber. A doctorblade bears against the engraved roll to scrape off excess coatingcomposition as the roll rotates. The applicator roll 47 rotates in apond 49 of coating retained by trough 50. A print film of adhesive iscarried by the roll 47 to the traveling web 43 and applied to the roughsurface designated at R on the side opposite the glazed surface G. Theweb 43, after passing the nip 46, is directed to the nip 36 and pressedinto contact with the traveling web 30. The rough side of the web 30,designated at R, is applied against the film of coating carried by theweb 43. The coating film is designated at 51 and, as shown, is somewhatexaggerated in thickness for the purposes of illustration.

The web emanating from the nip 36 is carried on the polished surface ofheated drum 38, and slow evaporation of any excess of moisture from theweb takes place on the drum 38. The drum 38 suitably is at a surfacetemperature of about 200 F. The dried sheet 52 may be dried to amoisture condition in which it is in equilibrium with the surroundingatmosphere or may be completely dried as desired. It is then passed fromdrum 38 over the guide roll 53 and then to windup. Again, it is to benoted that the rough sides of the glazed webs have been presented toeach other in a light pressure nip in such manner that there is nosubstantial compression of the webs and no impairment of the glazed websurfaces.

In the apparatus arrangement of FIG. 2, applicator roll 47, backing roll48, and drying drum 38 as well as the windup (not shown) are customarilyindependently appropriately driven, the remaining rolls being rotatablymounted.

' FIG. 4 illustrates a modification of the apparatus particularlyadapted for applying a portion of the coating to the rough side of eachof the dry traveling webs. As illustrated, a first pressure roll 57 anda second pressure roll 58 together form a pressure nip 59. The rolls 57,

8 58 are hard surfaced and are smooth. The dry preformed traveling web61 is fed over the roll 57 through the reverse roll coater 63, thecoating being applied to the rough side of the web designated at R. Theglazed side designated G contacts the roll 57. Similarly, a second dryweb 65 is fed through a reverse roll coater 67 to have adhesive appliedto its rough side, the glazed side G of this web contacting the smoothsurface of roll 58.

For the removal of the formed duplex sheet 69 from the laminatingapparatus, there is positioned around the periphery of one of the niprolls, a take-off roll 71. Thus, the sheet travels for a short distanceon the roll 57 prior to its removal to windup. Suitable support meanssuch as the carrier roll 72 provide for passage of the web to a reel(not shown).

In the arrangement of FIG. 4 the rolls 57, 58 are usually independentlydriven as are the rolls of the reverse roll coaters. It is to be notedthat the apparatus of FIG. 4 may be operated at any reasonable desiredspeed and usually will cooperate with the Yankee drier web formingequipment. Thus, the speed of the appparatus of FIG. 4 may be between500 and 3000 f.p.m., the latter speed being useful when the apparatus ofFIG. 4 cooperates with the output of 2 Yankee drier machines. On theother hand apparatus of FIG. 4 may, of course, be at a considerabledistance from the Yankees which form the webs; in fact, the webs 61, 65may be purchased, that is, they may be commercial machine glazed webs.

In connection with the apparatus of FIG. 4 it is to be particularlynoted that the rolls 57, 58' may but need not be heated, and the webs,while under tension sufficient to maintain them in good contact with therolls, are not adhered to the rolls.

Referring now to FIG. 3 and the duplex sheet in detail, there isillustrated a section of a paper web produced as by operation of theapparatus of FIGS. 1, 2 or 4. The numerals of FIG. 3, for convenience,correspond to those employed in FIG. 2. As shown in FIG. 3, the lowerweb 43 is bonded to the upper web 30 by a thin film of adhesive 51. Theadhesive, as will be noted, tends to follow, in large measure, the webto which it is applied. However, where the rough opposed surfaces of theglazed webs are closely adjacent or are in contact, the adhesive 51secures the ridges of the webs together as at 54 and bridges some of thespaces between the webs. Air spaces, as at 55, are defined by theopposed webs and the adhesive securing them. These air spaces are. ofcourse, relatively minute, the illustration in FIG. 3 being about 300times actual size. Accordingly, despite the presence of the air spaces,the webs are securely held together and resist delamination. Examinationof webs produced as described indicates the presence of about toadhesive contact points per lineal inch or 10,000 to about 22,500 persquare inch of sheet surface.

It is further to be noted that, although in large measure the adhesiveexhibits a preference for the web to which it is applied, the adhesivedoes spread to some extent on the rough side of each web.

The greatly enlarged section (FIG. 3) clearly illustrates the numerousbonding points of the adhesive despite the presence of the multiplicityof air spaces 55. The sheet of FIG. 3 made from two webs, each having abasis weight of 13 pounds, such as previously described in connectionwith FIG. 1, exhibits on its outer side good machine glazed finishes.Such finish on the Ingersoll glarimeter has a value of between about 40and 42. Additionally, such sheets have a size value of between 5 and 100seconds based on the Tappi Standard ink flotation test. The thickness ofthe sheet is about 0.0034". Since the basis weight is about 28 poundsper ream, the ratio of basis weight to thickness, which ratio is ameasure of apparent density, is about 8600. This compares with a valueof approximately 18,000 for a single printing sheet of comparableprinting quality in the light weight or catalog field.

If desired, a light coating may be applied to the duplex sheet followedby a very light calendering operation, that is, a pressure of less than300 pounds per inch of nip length. Such calendering pressure does reducethe thickness to some extent but the sheet will still exhibit more bulkthan a conventional sheet of similar basis weight which issupercalendered.

With other pulps, and employing apparatus as previously described inconnection with FIGS. 1, 2 or 4, the bulk is high but somewhat less thanthat achieved with the groundwool-sulfite combination. Thus, asemichemical pulp sheet having a basis weight of 36.2 pounds and athickness of 0.0033" exhibited a density of about 10,970. An all sulfitetype sheet having a basis weight of 34 pounds and a thickness of 0.0027exhibited an apparent density of about 12,592. -In contrast,commercially produced webs having a basis weight of 38-40 pounds totaland coatedabout 4 pounds of coat per sideexhibited apparet densities of19,000 to 21,000. In each of the foregoing, the data taken represents anaverage figure over a plurality of samples. In most cases the bulk ofsheets formed in accordance with the invention is very nearly twice thatof comparable printing papers employed in present commercial productionfor bookpaper, publication grade paper, and the like. In general, theapparent density is between 8,000 and 12,000 to No delamination occursin the printing operations even with tacky inks, and the sheet is thussatisfactory for the purpose for which is is designed. Of course, due tothe nature of the adhesives employed, no difiiculty is experienced withheat set inks.

While the printability of the described uncoated webs is excellent formany purposes, such as newsprint, catalog, and as an improvement inpresent uncoated bookpapers, it is frequently required that sheets forbook and publication be coated. The present duplex paper requires lesscoating material of the usual aqueous mineral coating type-approximatelyto /2 of that normally required. Further, only very light calendering isrequired to provide the paper on a comparable basis surface-wise withother heavily coated and calendered paper-s. As an illustration, thesheet produced in accordance with the example associated with FIG. 1(60% groundwood, etc.) may be coated with the usual aqueous mineralpigment (clay, starc-h-55% solids) to give a dry coat weight on eachside of about 4 pounds per ream.

Sheets produced in accordance with the invention compare favorably withsheets produced by conventional methods in all major respects, andparticularly, the new sheets demonstrate improved printability. In onesuch comparison the pulp employed contained by weight 27% sulfite, 18%kraft, and 55% bleached groundwood, with minor amounts of clay andphosphoric acid. The conventional, over an average of 40 tests, had abasis weight when coated on both sides with the usual aqeous mineralpigment coating composition of 44.8 pounds. The coating wasapproximately 5 pounds per side. Such sheets were supercalendered aftercoating; the apparent density was 19,000, the thickness 0.0023, theopacity (Bausch and Lomb) 93, and the gloss (Ingersoll glarimeter) 43,while the brightness (GE) was 73. While the gloss and opacity weresomewhat higher than that of the duplex sheet produced from the samepulp, the printability of the duplex sheet was superior.

The comparative duplex sheet produced from this same furnish and asdescribed in connection with FIG. 1 had a basis weight of only 30.9pounds, was uncoated, and had a thickness of 0.0033". The apparentdensity of this duplex material was only 9400-less than /2 of that ofthe coated product. Opacity (Bausch and Lomb) of the duplex sheet was84, GE brightness 71.2, and Ingersoll glarimeter value 38.6. Theimproved half-tone print- 10 ability of the uncoated sheet over thecoated material is reflected by very uniform ink absorbency which isascribed to the unformity of formation, the levelness of surfaceachieved by machine glazing, and its retention throughout the describedprocedure, as well as the very low apparent density of the duplex sheet.

Similar comparative tests employing very high percent groundwoodfurnishesa-s in newspaper furnishes having 75 to groundwoodclearlyindicate that the printability is upgraded by the use of the duplexsheet.

Single nip calendering under relative light pressure of about 200-300pounds per lineal inch further increases printability and is usefulwhere some loss of bulk may be tolerated.

In the procedures previously described, the wire side of the web hasbeen the side which has been machine glazed. This is normally therougher side of the Web and in conventional machine glazing is the sidepresented to the Yankee drier. If desired, the felt side of the web maybe the glaze side and the wire side may be the rough web side forlamination. With the conventional commercial Yankee machines, thisrequires turning the web prior to presentation to the Yankee. Such maybe readily accomplished and some benefit results as the felt side, beingsomewhat more smooth initially, glazes more readily. However, eitherside may be utilized as the glazed side in the practice of theinvention.

The tensile strength and M.I.T. fold values of the duplex sheet areimproved over the corresponding single sheets produced from the samefurnish; the tear strength of the duplex sheets is usually improved orat least comparable to that of a single sheet of a corresponding basisweight; these factors together with the bulky, low apparent densitycharacteristic at good solids and half-tone printability, and Bausch andLornb opacity values of at least 80 provide for lower basis weights inhigh quality paperan important factor in publication grade and bookpapers. A particular advantage is that such paper permits of lowermailing costs due to improved quality with decreased weight. Also, thecost of manufacturing operations such as coating and supercalenderingmay be eliminated.

In the specification and appended claims, the term dry as applied to thewebs is intended to indicate a normally dry Web in equilibrium with asurrounding atmosphere, such webs normally containing 56% moisture; thisis in contrast to the term bone dry, which indicates a web substantiallyfree of moisture. Also, the term bonded areas or bonded contact pointsis employed to designate the contacts between opposing webs, that is,the web high points which project and are secured by adhesive.

It will be understood that this invention is susceptible to modificationin order to adapt to different usages and conditions and, accordingly,it is desired to comprehend such modifications within the invention asmay fall within the scope of the appended claims.

What is claimed is:

1. A printing paper in which is an uncalendered sheet having a thicknessin the range of about 0.003" to about 0.005" and comprising a pair ofsuperimposed sized cellu losic paper webs, each web having a machineglazed surface and a rough surface, the rough surfaces comprising ridgesand valleys and the machine glazed surfaces being substantially planar,said webs having their rough surfaces confronting, and a thin layer ofadhesive bonding ridges of the rough surfaces together at their areas ofcontact, said sheet having a basis weight of between about 28 to 45pounds (25" x 38" x 500 sheets) including an pounds, a proportion of thevalleys of the rough surfaces being free of adhesive whereby saidvalleys define air spaces which contribute to the bulk of the sheet,said glazed surfaces each having an Ingersoll glarimeter value of atleast 38, said sheet having an apparent density of between about 8,000and 13,000 and a Bausch and Lomb opacity of at least 80.

2. Printing paper as claimed in claim 1 and wherein the bonded contactareas of the webs are in the range of 10,000 to 22,500 per square inch.

3. In theproduction of printing paper having a thickness in the range of0.003" and 0.005 and an apparent density of about 8,000 to 13,000, thesteps of passing a first sized cellulosic base web having rough sidesand a basis weight of about 10-20 pounds per ream in the wet state to aYankee drier, leveling one side of said base web by pressing said webwhile wetted against the smooth surface of a Yankee drier, drying theweb on said smooth surface to provide a machine glaze surface on the webon the drier side while the other side remains rough, passing the webwithout removal from the drier to a nip formed between the drier and abacking roll, advancing a second and similar dry sized cellulosic paperweb having one machine glaze surface and one rough surface to the nip insuch manner that rough surfaces of the machine glazed webs are inconfronting relation, applying a thin film of an adhesive to said secondweb as it advances to said nip,- rolling the said webs with the adhesivethercbetween into a unitary'sheet without impairment of the machineglaze surfaces of the webs, passing the unitary sheet on the Yankeedrier for a short distance beyondthe nip, and then removing the sheet ina dried condition from said drier. 1

References Cited by the Examiner UNITED STATES PATENTS 1,986,961 1/35Dodge.

FOREIGN PATENTS 1,049,224 1/ 59 Germany.

515,011 11/39 Great Britain. 572,944 10/45 Great Britain. 7

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner.

1. A PRINTING PAPER IN WHICH IS AN UNCALENDERED SHEET HAVING A THICKNESSIN THE RANGE OF ABOUT 0.003" TO ABOUT 0.005" AND COMPRISING A PAIR OFSUPERIMPOSED SIZED CELLULOSIC PAPER WEBS, EACH WEB HAVING A MACHINEGLAZED SURFACE AND A ROUGH SURFACE, THE ROUGH SURFACES COMPRISING RIDGESAND VALLEYS AND THE MACHINE GLAZED SURFACES BEING SUBSTANTIALLY PLANAR,SAID WEBS THEIR ROUGH SURFACES CONFRONTING, AND A THIN LAYER OF ADHESIVEBONDING RIDGES OF THE ROUGH SURFACES TOGETHER AT THEIR AREAS OF CONTACT,SAID SHEET HAVING A BASIS WEIGHT OF BETWEEN ABOUT 28 TO 45 POUNDS (25" X38" X 500 SHEETS) INCLUDING AN POUNDS, A PROPORTION OF THE VALLEYS OFTHE ROUGH SURFACES BEING FREE OF ADHESIVE WHEREBY SAID VALLEYS DEFINEAIR SPACES WHICH CONTRIBUTE TO THE BULK OF THE SHEET, SAID GLAZEDSURFACES EACH HAVING AN INGERSOLL GLARIMETER VALUE OF AT LEAST 38, SAIDSHEET HAVING AN APPARENT DENSITY OF BETWEEN ABOUT 8,000 AND 13,000 AND ABAUSCH AND LOMB OPACITY OF AT LEAST
 80. 3. IN THE PRODUCTION OF PRINTINGPAPER HAVING A THICKNESS IN THE RANGE OF 0.003" AND 0.005" AND ANAPPARENT DENSITY OF ABOUT 8,000 TO 13,000, THE STEPS OF PASSING A FIRSTSIZED CELLULOSIC BASE WEB HAVING ROUGH SIDES AND A BASIS WEIGHT OF ABOUT10-20 POUNDS PER REAM IN THE WET STATE TO A YANKEE DRIER, LEVELING ONESIDE OF SAID BASE WEB BY PRESSING SAID WEB WHILE WETTED AGAINST THESMOOTH SURFACE OF A YANKEE DRIER, DRYING THE WEB ON SAID SMOOTH SURFACETO PROVIDE A MACHINE GLAZE SURFACE ON THE WEB ON THE DRIER SIDE WHILETHE OTHER SIDE REMAINS ROUGH, PASSING THE WEB WITHOUT REMOVAL FROM THEDRIER TO A NIP FORMED BETWEEN THE DRIER AND A BACKING ROLL, ADVANCING ASECOND AND SIMILAR DRY SIZED CELLULOSIC PAPER WEB HAVING ONE MACHINEGLAZE SURFACE AND ONE ROUGH SURFACE TO THE NIP IN SUCH MANNER THAT ROUGHSURFACES OF THE MACHINE GLAZED WEBS ARE IN CONFRONTING RELATION,APPLYING A THIN FILM OF AN ADHESIVE TO SAID SECOND WEB AS IT ADVANCES TOSAID NIP, ROLLING THE SAID WEBS WITH THE ADHESIVE THEREBETWEEN INTO AUNITARY SHEET WITHOUT IMPAIRMENT OF THE MACHINE GLAZE SURFACES OF THEWEBS, PASSING THE UNITARY SHEET ON THE YANKEE DRIER FOR A SHORT DISTANCEBEYOND THE NIP, AND THEN REMOVING THE SHEET IN A DRIED CONDITION FROMSAID DRIER.